An oral appliance fixed to lower jaw teeth comprises a body having a shape corresponding to a teeth arrangement, a lower teeth recess formed in a lower portion of the body to allow lower jaw teeth to be inserted therein, an upper teeth recess formed in an upper portion of the body to allow upper jaw teeth to be inserted therein and allowing the lower jaw to move backward by a predetermined distance, and an upper teeth guide part formed to extend from the body to face a side of the upper jaw teeth. The lower teeth recess further comprises a lower molar fixing recess formed to strongly engage with at least a portion of left and right molars of the lower jaw teeth by frictional force, and a lower incisor recess formed to allow other lower jaw teeth except the left and right molars to position therein.
